Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Sensorineural hearing loss is due to the absence or destruction of the hair cells in the cochlea that transduce sound signals into nerve impulses. Various hearing prostheses are commercially available to provide individuals suffering from sensorineural hearing loss with the ability to perceive sound. One example of a hearing prosthesis is a cochlear implant.
Conductive hearing loss occurs when the normal mechanical pathways that provide sound to hair cells in the cochlea are impeded, for example, by damage to the ossicular chain or the ear canal. Individuals suffering from conductive hearing loss may retain some form of residual hearing because the hair cells in the cochlea may remain undamaged.
Individuals suffering from conductive hearing loss typically receive an acoustic hearing aid. Hearing aids rely on principles of air conduction to transmit acoustic signals to the cochlea. In particular, a hearing aid typically uses an arrangement positioned in the recipient's ear canal or on the outer ear to amplify a sound received by the outer ear of the recipient. This amplified sound reaches the cochlea causing motion of the perilymph and stimulation of the auditory nerve.
In contrast to hearing aids, which rely primarily on the principles of air conduction, certain types of hearing prostheses commonly referred to as bone conduction devices, convert a received sound into vibrations. The vibrations are transferred through the skull to the cochlea causing generation of nerve impulses, which result in the perception of the received sound. In some instances, bone conduction devices can be used to treat single side deafness, where the bone conduction device is attached to the mastoid bone on the contra lateral side of the head from the functioning “ear” and transmission of the vibrations is transferred through the skull bone to the functioning ear. Bone conduction devices can be used, in some instances, to address pure conductive losses (faults on the pathway towards the cochlea) or mixed hearing losses (faults on the pathway in combination with moderate sensoneural hearing loss in the cochlea).
Another type of device that treats conductive hearing loss is a direct acoustic cochlear implant (DACI).
A scenario exists where recipients of cochlear implants (cochlear implant users) have residual hearing in the non-implanted ear and/or in the implanted ear, such that stimulation of the ear having residual hearing with an acoustic hearing aid can evoke a hearing percept.
Bimodal auditory prosthesis systems include an auditory prosthesis fitted to the right ear of a recipient and an auditory prosthesis fitted to the left ear of a recipient, where there is residual hearing in at least one of the ears, and the prostheses are of different types (e.g., a cochlear implant and an acoustic hearing aid). In at least some situations, the auditory prosthesis fitted to each ear operates on a different principle of operation (e.g., one is a cochlear implant and the other is an acoustic hearing aid). These devices are typically developed separately and are fitted separately (e.g., parameters of the devices are adjusted based on features associated with the individual recipient, thereby “fitting” the prosthesis to the individual recipient). This results in very different growth of loudness with level with respect to the two separate ears, potentially leading to decreased wearing comfort and suboptimal perception of interaural loudness differences.
Hybrid auditory prosthesis systems include two different stimulation devices fitted to the same ear (e.g., a cochlear implant and an acoustic hearing aid).